If someone quits or is fired from his job but indicates on his resume/through the interview process that he is still working at his former company, what is the likelihood of the new potential employer finding out about this during or after the interview process?

Depends on the company. Some companies give offers contingent on a background check. The background check may include confirmation of employment dates at a particular company. Let's say you claim that you worked at a company through July 2013, but your former employer says you left on May 2013 -- you'll have a lot of explaining to do.

There's a chance your current employer will find out, and it's not a chance worth taking. Be an ethical person, even if it means hurting your future opportunities. At the end of the day, your character is more important than your employment status. If you're really worth the amount of money you wish to make, then learn to negotiate really well.

Thanks for the A2A. I think you can guess my answer, because so many people have already said the same thing: namely, don't do it! Don't do it because it's wrong. It's as simple as that. It's wrong. You're lying. Don't do it because it's very easy for you to be caught. Don't do it because lying is actually harder than just telling the truth. Once you start lying, you have to keep track of all your lies. The lies are going to pile on top of each other, and pretty soon it will be hard to remember what you said to each person. I also wish to address a couple of points you made: You think the risk is justified because it is "easier to find a job when already in possession of one..."  Yes, it does look better if you are already employed, but this still doesn't justify the risk of lying. You feel that being currently employed "demonstrates value/pre selection and increases leverage to ask for more money..." You are worth what you are worth. Period. I wouldn't offer someone more money simply because he or she is already employed. You feel that "it's easier to control the references that have to be given". You are always in control of the references you supply. So, again, don't lie! It's not worth it.

Pretty darn likely. The people below cover all of the reasons. if one was looking for a way around something like this, HYPOTHETICALLY, you could ask someone at the old job to vouch for you, and make sure you give the new employer their number, but the things that could go horribly, horribly wrong are pretty numerous. Unless you are a gambling man...

At my last management job at a large company it was part of the hiring process to verify the previous employers, the dates employed and titles.  Any discrepancy would have resulted in any offer being rescinded. When we hired contractors through an agency, part of our contract with the agency was that they conduct these for us. I fired an agency that didn't do it in one case. It's a small world. I received a resume from someone I knew , who claimed a job title I didn't think he had. I personally verified with someone who had been his supervisor. Now both of us know he outright lied on his resume. I confronted him and he denied it. He was clearly lying again. In the end he really damaged his reputation. I'd have to say the chance s of getting caught are high and it will not be pleasant when it happens. And it may follow you for some time after.

At this point, you can't get a job fixing roller-skates without a background check which will, among other things, likely reveal the actual "dates of employment" at your old job. And even if the company you're applying to doesn't do an "official" background check, they can easily do informal reference checks with any of their LinkedIn contacts that work at your former company and "figure it out." This would expose you to the embarrassment (and financial loss) of a withdrawn offer. Worse, if they don't discover it until after you work there you'll almost certainly be fired on the spot, and then have to go back to job-searching with the albatross of having been fired from your last two consecutive jobs, which will do incalculable damage to your reputation and employability. So to answer your question directly: What are the odds of an employer finding out you've lied about your employment status after an offer is extended? Very good and rapidly approaching 100%. Only the least responsible organization isn't doing some kind of background check at this point, and even the laziest background checker knows to ask your former employer when you left.